1. Field of the Invention
The present invention relates to a magneto-optical recording medium having a recording layer formed on a substrate thereof on which grooves and embossed pits are formed.
2. Description of Related Art
As one of magneto-optical recording media, a magneto-optical disc is schematically illustrated in FIG. 1. As shown, the magneto-optical disc has a recording track or tracks formed spirally or concentrically thereon. Such a magneto-optical disc has on a substrate thereof a recording layer of which the Kerr effect is used to write and/or read information signal to/from the magneto-optical disc.
It should be noted that in the plane of the recording layer of the magneto-optical recording medium, a direction perpendicular to the recording track will be referred to herein as "x-direction" while a direction tangential to the recording track will be referred to as "y-direction". Therefore, in the magneto-optical disc shown in FIG. 1, the radial direction of the magneto-optical disc is the x-direction while the tangential direction of the recording track is the y-direction.
To read information signal from the magneto-optical disc, a linearly polarized laser light is irradiated from a laser source 100 having a semiconductor laser or the like, and it is focused on the recording layer of a magneto-optical disc 102 by an objective lens 101 as shown in FIG. 2. A return light from the recording layer is taken out through a first polarizing beam splitter 103, and passed through a phase compensator plate 104 and 1/2-wavelength plate 105. It is thus incident upon a second polarizing beam splitter 106. A laser light transmitted through the second polarizing beam splitter 106 is detected by a first photodiode 107, while a light reflected by the second polarizing beam splitter 106 is detected by a second photodiode 108.
The return light from the magneto-optical disc 102 has a plane of polarization thereof rotated due to the Kerr effect correspondingly to the information signal recorded at the recording layer. As the plane of polarization of the return light is rotated, the output from the first photodiode 107 which detects a polarized component of the return light having been transmitted through the first polarizing beam splitter 106 is increased or decreased depending upon the rotating direction of the return light plate of polarization as shown in FIG. 3A. Similarly, the output from the second photodiode 108 which detects a polarized component of the return light having been reflected by the second polarizing beam splitter 106 is increased or decreased depending upon the rotating direction of the return light plane of rotation as shown in FIG. 3B.
When the output from the first photodiode 107 is increased, the output from the second photodiode 108 is decreased. Reversely, when the output from the first photodiode 107 is decreased, the output from the second photodiode 108 is increased. Therefore, as shown in FIG. 3C, a differential is determined between the output from the first photodiode 107 and that from the second photodiode 108 to detect the rotated status of the plane of polarization of the return light, thereby detecting an information signal. That is, an information signal recorded at the recording layer of the magneto-optical disc 102 is detected and reproduced as a signal of a differential between the output from the first photodiode 107 and that from the second photodiode 108. The differential signal will be referred to as "MO signal" hereinunder.
Recently, magneto-optical discs adopting a recording format called "ZCAV (zone constant angular velocity) format" have been developed to attain a further higher recording density.
The ZCAV format is such that to eliminate a difference in linear density between lead-in and lead-out areas, the recording area of a magneto-optical disc is divided into a plurality of concentric bands 110a, 110b, 110c and 110d, as shown in FIG. 4, in each of which a fixed frequency is used for write and/or read of information signal. That is, in a ZCAV-formatted magneto-optical disc, areas where a basic frequency is used to record information signal are grouped as one band and thus the whole surface of the magneto-optical disc is divided into some bands different in recording frequency from one another.
In the ZCAV format, each of the bands is divided into sectors 111 disposed to have the same angle so that a fixed frequency can be used for read and/or write of information signal from/to the sectors 111 in the same band. Therefore, the sectors 111 are disposed in a circumferential series in the same band, but sectors in one band are displaced from those in the other band adjacent to the one band. That is, the ZCAV format provides sectors 111 formed in one band and circumferentially displaced from those formed in a band adjacent to the one band.
Usually, in a magneto-optical disc, the sector 111 has an embossed pit area 112, in the leading portion thereof, where there are pre-formed embossed pits of which the convex-concave pattern indicates information such as address, and a magneto-optical recording area 113, in the other portion thereof, where no embossed pits are formed and data is written by magneto-optical recording.
From the embossed pit area 112, a signal indicative of a change in reflected quantity of light is read, and a signal is read by the Kerr effect from the magneto-optical recording area 113 where data is written by the magneto-optical recording.
In the ZCAV format, the sectors 111 formed in one band are circumferentially displaced from those in a adjacent band. Thus, at the boundary between the bands, the embossed pit area 112 disposed in the leading portion of the sector 111 will be adjacent to the magneto-optical recording area 113 in the adjacent track as shown in FIG. 5 being a view, enlarged in scale, of the portion A in FIG. 4.
Usually, as a substrate of magneto-optical disc, a disc-like substrate formed by injection molding of a polymeric material such as polycarbonate is used because it will contribute very much to a reduced cost in mass production of the magneto-optical disc. Generally, the substrate formed from such a polymeric material has such a photoelasticity that a ZCAV-formatted magneto-optical disc is disadvantageous as in the following:
In the ZCAV-formatted magneto-optical disc, the embossed pit area 112 and magneto-optical recording area 113 are adjacent to each other at a boundary as shown in FIG. 5. When the embossed pit area 112 adjoins the magneto-optical recording area 113, stresses B developed around embossed pits 114 will cause a local change of the optical characteristic of the magneto-optical. In a substrate having a large coefficient of photoelasticity like the polycarbonate substrate widely used as the substrate of magneto-optical disc, the stresses around the embossed pits 114 will cause a local change of the optical characteristic of the magneto-optical disc.
Such a local change of the optical characteristic will cause a distortion of MO signal. That is to say, since the optical characteristic is locally changed at the boundary between the bands in the ZCAV formatted magneto-optical disc, the signal distortion at the band boundary will be a cross-talk which will deteriorate the quality of MO signal.
Such a cross-talk will take place not only at one adjacent directly to the embossed pit area 112, of the recording tracks in the magneto-optical recording area 113, but at a recording track not adjacent directly to the embossed pit area 112 within a range affectable by the local change of optical characteristic. Thus, even at a recording track 10 tracks apart from the embossed pit area 112, the cross-talk will take place as the case may be.
The magnitude of the cross-talk caused by the aforementioned local change of optical characteristic of the substrate depends upon a phase difference caused by an optical system of a magneto-optical disc drive, macroscopic birefringence at a disc substrate, etc. The intensity of MO signal depends upon a phase difference caused due to the magneto-optic Kerr effect of the recording layer, for example, in addition to the above-mentioned phase difference and birefringence. In a magneto-optical disc having grooves and lands formed along the recording tracks, the intensity of MO signal depends upon a phase difference caused by the effect of diffraction at the groove and land.
As in the above, the magnitude of a cross-talk in the MO signal depends upon various factors. Therefore, the magneto-optical disc should be considered from various aspects to suppress a cross-talk caused by the aforementioned local change of optical characteristic of the substrate in the ZCAV-formatted magneto-optical disc and assure a sufficiently high output of reproduction from the disc.